1. Field Of The Invention
The present invention relates generally to the separation of liquid-liquid mixtures, and more particularly but not by way of limitation to the separation of oil from oily water produced at an oil well production site.
2. Description Of The Prior Art
In a typical oil well production operation, the amount of produced water increases as the field matures. In some operations, the bulk of the volume of produced fluids may be water. Although there is no direct economic incentive, recent tightening of government regulations in various parts of the world regarding the amount of oil in discharged waters has increased interest in improving and optimizing oily water separators.
This problem is particularly acute on offshore production platforms. Size and weight limitations on separation equipment limit the available options. Furthermore, on floating offshore platforms, the movement of the platform may affect the performance of some traditional types of separating equipment.
The most traditional scheme utilized for cleanup of oily water on offshore production platforms includes a weir type primary separator which allows the oily water to stand for a period of time such that free oil can accumulate at the top thereof and pass over a weir, with the cleaner stream then being drawn off from the primary separator and directed to a flotation type secondary separator. The flotation type secondary separator is very large, on the order of the size of a large room, and is motion sensitive. A level control valve is disposed between the primary and secondary separators and is operably associated with a level sensing device in the primary separator to account for varying input flow and to maintain the appropriate fluid level in the primary separator for operation of the weir.
As offshore fields mature and the volume of water production becomes greater and greater, traditional systems like that just described become less and less practical. Furthermore, the motion sensitive nature of the secondary separators utilized are particularly unsuitable to floating platforms such as tension leg platform designs which have come into use in recent years.
More recently, the use of hydrocyclone separators as a substitute for the flotation type secondary separator has been proposed. Early work on the design of hydrocyclones suitable for separating entrained oil droplets from oily water is found in a paper of Colman et al., entitled "Hydrocyclones for Oil/Water Separation" presented at the International Conference on Hydrocyclones, held Oct. 1-3, 1980, at Churchill College, Cambridge, UK. In addition to discussing suitable designs for such hydrocyclones, Colman et al. discloses with reference to FIG. 3 thereof an isokinetic sampling technique for sampling a flowing stream of oily water and analyzing the droplet size distribution therein. The sampling technique of Colman et al. was limited in that it was purely a laboratory system operating at low pressures. It was not operable over a wide range of flow rates, and it sampled the flowing stream at only a single position on its cross section.
Subsequent work taking this hydrocyclone technology into the field and actually using it to clean up oily water on an offshore production platform is reported in a paper by Meldrum, entitled "Hydrocyclones: A Solution to Produced Water Treatment", Paper No. OTC 5594 presented at the 19th Annual Offshore Technology Conference in Houston, Tex., on Apr. 27-30, 1987.
Meldrum reported the successful application of hydrocyclone separators in a situation in the Murchison field where the oily water was available from the primary or first stage separator at a pressure sufficient to drive the hydrocyclones. The hydrocyclones in Meldrum's application were installed immediately downstream of the primary or first stage separator, but upstream of the separator level control valves.
Meldrum also reported on a second application of this hydrocyclone technology to an offshore tension leg platform in the Hutton field where the oily water was available at a significantly lower initial pressure. The pressure was sufficient, however, to provide relatively satisfactory performance of the hydrocyclones, although the results were less successful than had been achieved in the Murchison field where the oily water was available at higher pressures.
As part of the work reported by Meldrum on the Hutton field where low pressure of the oily water provided less than optimum conditions for use of the hydrocyclones, Meldrum reported on tests utilizing pumps to boost the pressure of the oily water before directing it to the hydrocyclones. Meldrum's combination of pumps and hydrocyclones was unsuccessful in satisfactorily cleaning up the oily water. Meldrum recognized that the basic problem was at least in part that the pumps and other equipment upstream of the hydrocyclone were breaking up the oil droplets into smaller sizes which were more difficult for the hydrocyclone to separate.
The pump utilized by Meldrum in his tests was a Moyno two-stage progressive cavity pump, of the type which is described below in the specification of this application as a "large progressive cavity pump". Meldrum apparently was operating this pump at a relatively low percentage of its flow capacity. Although Meldrum had not discovered a successful combination of pump and hydrocyclone for oily water treatment, he did hypothesize that such a combination could be achieved, and suggested with regard to the pump that it was desirable to achieve increased pump discharge pressures "without increasing speed of rotation" of the pump. This was in line with traditional theories of "low shear" pumps which involved the use of oversized pumps turning at relatively low speeds and thus operating at a relatively low percentage of their maximum capacity in order to minimize "shear" of the fluids being pump.
Thus, the prior art recognized the need for a pump and hydrocyclone combination capable of cleaning low pressure oily water.